The invention relates to an MR imaging method in which a number of MR data sets is combined so as to form an image of the nuclear magnetization distribution in an examination zone while utilizing the fact that a spatial sensitivity profile of a receiving element used for the acquisition of the relevant MR data set is associated with each MR data set. The invention also relates to an MR apparatus for carrying out the method and to a computer program for controlling an MR apparatus of this kind.
In MR imaging is common practice to pick up the magnetic resonance signal as a voltage that is induced, under the influence of a suitable sequence of RF pulses and gradient pulses in the time domain, into a coil that encloses the examination zone. The actual image reconstruction is performed by Fourier transformation of the time signals. The number, the spacing, the duration and the strength of the gradient pulses used define the sampling of the reciprocal k space that defines the volume to be imaged (Field of View or FOV) as well as the image resolution. The requirements imposed in respect of the image format and the image resolution define the number of phase encoding steps and hence the duration of the imaging sequence. The foregoing leads directly to one of the essential drawbacks of magnetic resonance tomography, because the acquisition of an image of the complete examination zone with a resolution that suffices for diagnostic purposes usually requires an undesirably long period of time.
A large number of technical developments in the field of magnetic resonance tomography aim to achieve a drastic reduction of the image acquisition times. Hardware developments that enable an as fast as possible switching of the magnetic field gradients have now reached the limits of technical feasibility as well as the limits of what a patient can be expected to bear physiologically. For a number of applications that include notably interventional MR imaging, however, the acquisition times are still too long.
It seems that the existing technical and physiological speed limits of conventional Fourier imaging can be overcome by the recently emerging parallel MR methods such as, for example the SMASH (Simultaneous Acquisition of Spatial Harmonics) technique or the SENSE (Sensitivity Encoding) technique. These techniques are based on the insight that the spatial sensitivity profile of the receiving elements (resonators, coils, antennae) impress on the magnetic resonance signals positional information that can be used for the image reconstruction. When a plurality of separate receiving elements, each having a different spatial sensitivity profile, is used in parallel, the acquisition time required for an image can be reduced by combination of the respective detected magnetic resonance signals; in comparison with customary Fourier reconstruction a reduction is achieved by a factor which is generally equal to the number of receiving elements used (see Pruessmann et al., Magnetic Resonance in Medicine 42, pp. 952 to 962, 1999).
An MR imaging method of the kind set forth is described, for example in WO 99/54746. According to said known method first a plurality of MR data sets is acquired while utilizing a plurality of receiving coils. The sensitivity of each of the receiving coils is dependent on the distance between the nuclear magnetization and the relevant coil. The combining of the individual image data sets utilizes the spatial sensitivity profiles so as to reconstruct an image of the examination zone that is larger than the scanning zone predetermined by the pulse sequence used.
The use of the SENSE method is particularly advantageous when the MR signals are picked up by means of surface coils that are arranged directly on the patient. Coils of this kind have a severely inhomogeneous spatial sensitivity profile; this represents an advantage for the image reconstruction by means of SENSE. For MR angiography, for example, a flat RF coil can be arranged underneath the back of a patient and another flat RF coil can be positioned on the thorax. Because the spatial sensitivity profiles of the receiving coils must be accurately known in order to carry out the SENSE method, the imaging operation starts with a so-called reference measurement. During this reference measurement the full FOV is additionally scanned by means of a whole body coil that has a spatially constant sensitivity profile. The spatial sensitivity profiles of the surface coils used are then determined by comparison, that is, by relating the image data acquired by means of the surface coils to the data of the whole body coil. A comparatively low image resolution can be used for the determination of the spatial sensitivity profiles. Therefore, the reference measurement can be performed very quickly. During the subsequent image acquisition a reduced FOV is used, the complete image being reconstructed while utilizing the sensitivity profiles determined, that is, by combining the MR data simultaneously acquired by means of the two surface coils. In comparison with conventional Fourier imaging, the image acquisition times have thus been reduced by a factor of 2 for the same image format and the same image resolution.
However, the known SENSE imaging method has the drawback that in the course of time changes are liable to occur in the position in space of the receiving coils. It is notably when surface coils are arranged directly on the patient that displacements of the receiving coils are unavoidable because of motions of the patient. Such a displacement of the receiving elements changes the associated spatial sensitivity profiles to such an extent that the image reconstruction in conformity with the SENSE method is subject to errors.